In order to meet the emission level requirements, for industrial low emission gas turbine engines, staged combustion is required in order to minimise the quantity of the oxides of nitrogen (NOx) produced. Currently the emission level requirement is for less than 25 volumetric parts per million of NOx for an industrial gas turbine exhaust. The fundamental way to reduce emissions of nitrogen oxides is to reduce the combustion reaction temperature and this requires premixing of the fuel and all the combustion air before combustion takes place.
It is known to provide gas turbine engine combustion chambers which have staged combustion to minimise nitrous oxide (NOx) emissions. Our UK patent no 1489339 discloses two stages of fuel injection in a gas turbine engine combustion chamber to reduce NOx. Our International patent application No. 9207221, published Apr. 30, 1992 discloses two and three stages of fuel injection in a gas turbine engine combustion chamber. In staged combustion, all the stages of combustion seek to provide lean combustion and hence the low combustion temperatures required to minimise NOx. The term lean combustion means combustion of fuel in air where the fuel to air ratio is low, i.e. weaker than the stoichiometric ratio. A problem with this arrangement is that it does not minimise the emission of nitrous oxide (NOx) to below the current emission level requirement of 25 volumetric parts per million of NOx for an industrial gas turbine exhaust throughout the range 40% to 100% power of the gas turbine engine, with simultaneous low emission levels of carbon monoxide. Furthermore this arrangement requires accurate knowledge of the fuel composition, and the air humidity to control the relative proportions of fuel and air supplied to the combustion chamber in order to minimise the emissions of NOx. Additionally the fuel valves require precise calibration in order to achieve this.
It is also known to provide gas turbine engine combustion chambers which have a plurality of catalytic combustion zones arranged in series to minimise nitrous oxide (NOx) emissions. One known arrangement is described in our United Kingdom patent application 2268694A, published Jan. 19, 1994.
A problem with this arrangement is that it does not fit into the space available, and it may require staged fuelling between the catalytic combustion zones.